1. Field of the Invention
The present invention relates to a single input level shifter and a thin film transistor (“TFT”) liquid crystal display (“LCD”) device using the same, and more particularly, to a single input level shifter that can perform a stable level-shifting operation without increasing its area when used with TFTs having a variety of different characteristics, and a TFT LCD device using the same.
2. Description of the Related Art
As personal computers and television sets get smaller, lighter, and thinner, the demand for smaller, lighter, and thinner display devices increases. In order to meet this demand, flat panel display devices such as liquid crystal display (“LCD”) devices have been developed to replace cathode ray tubes (“CRT”).
LCD devices are display devices that apply an electric field to a liquid crystal material injected between two substrates and having an anisotropic dielectric constant. LCD devices also adjust the intensity of the electric field and adjust the amount of light transmitted onto the substrates, thereby generating a desired image signal. Examples of common LCD devices include portable flat panel display devices, and thin film transistor (“TFT”) LCD devices that use TFTs as switching elements.
In general, a TFT LCD device includes a display unit having a plurality of gate lines that transmit a scan signal, a plurality of data lines that transmit gray-scale voltages in response to an image data signal, where the plurality of data lines are isolated from and cross the plurality of gate lines, and a plurality of pixels arranged in a matrix and having switching elements connected between the plurality of gate lines and the plurality of data lines. The TFT LCD further includes a gate driving unit that provides the scan signal to the gate lines, a data driving unit that provides the gray-scale voltages to the data lines, a timing control unit that provides a logic signal, and a level shifter that level-shifts the logic signal and transmits the level-shifted logic signal to the gate driving unit or the data driving unit.
Here, the level shifter receives a digital logic signal from the external timing control unit and amplifies the signal so as to drive the gate driving unit or the data driving unit.
A conventional level shifter for an LCD device will now be described with reference to FIG. 1. The level shifter includes metal oxide semiconductor (“MOS”) transistors made of layers of silicon, an insulting oxide layer, and a metal gate. An MOS transistor is a voltage-controlled switch that has three connection points including a source, a drain, and a gate. A p-channel type, or PMOS transistor for short, and an n-channel type, or NMOS transistor for short, are made from materials with different affinities for electrons. Conduction for the PMOS transistor is based on holes, and conduction for the NMOS transistor is based on electrons.
In a case where an input signal IN input to the level shifter of FIG. 1 is low, a PMOS transistor MP5 is turned on and an inverted signal INB of the input signal IN, i.e., a high level, is applied to a PMOS transistor MP2 and an NMOS transistor MN2. As a result, the PMOS transistor MP2 is turned off, the NMOS transistor MN2 is turned on, and a ground voltage is provided as an output signal OUT. On the other hand, when the input signal IN is high, an NMOS transistor MN3 is turned on, and the inverted signal INB applied to the PMOS transistor MP2 and the NMOS transistor MN2 is low. As a result, the PMOS transistor MP2 is turned on, the NMOS transistor MN2 is turned off, and, in an ideal situation, a supply voltage is provided as an output signal OUT. Thus, the level shifter provides a voltage signal, i.e., the supply voltage, which has a larger voltage than the input signal.
However, the level shifter of FIG. 1 comprises a plurality of TFTs that can perform a stable level-shifting operation only when the size of the high-level voltage of the input signal is equal to or greater than a predetermined voltage. Due to a high threshold voltage, a low field effect mobility μ, or large subthreshold swing, the TFTs operate stably only at voltages above the threshold voltage. Thus, when the voltage level of the input signal is equal to or less than the threshold voltage, an NMOS transistor is not sufficiently turned on and cannot invert the input signal. When the input signal cannot be inverted, the output signal OUT of the level shifter is maintained at a high level or a low level, but the voltage level of the output signal OUT is less than that of the supply voltage. In addition, the level-shifting operation may vary depending on the field effect mobility or subthreshold swing, and the deviation of the threshold voltage.
In U.S. Pat. No. 6,404,230, a level shifter for an LCD device is disclosed that level-shifts an input signal by separately applying the input signal and an inverted signal of the input signal. However, since an interconnection for transmitting the input signal and an interconnection for transmitting the inverted signal of the input signal are required, the area of the level shifter increases, and the number of output terminals of a timing control unit increases.
In Korean Patent Laid-open Publication No. 2003-0051920, a level shifter for an LCD device is disclosed that level-shifts an input signal by separately applying an input signal and a reference voltage. However, since an interconnection for transmitting a reference voltage is coupled with an interconnection for transmitting other voltage signals, and a stable reference voltage cannot be transmitted, malfunctions may occur.